|Year : 2002 | Volume
| Issue : 4 | Page : 261-263
Gene therapy in medicine
J Ray, K Ray
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Ray J, Ray K. Gene therapy in medicine. Indian J Ophthalmol 2002;50:261-3
Gene therapy intends to treat or alleviate disease by genetically modifying the cells of the patients. It is a promising endeavour for the treatment of disease in the 21st century. One of the goals of gene therapy is to develop treatment for diseases for which there is no classical treatment available. The therapy aims to treat genetic disease by delivering normal copies of the defective gene. To cope with wide variation in the molecular basis of diseases, different gene therapy strategies are planned and executed.
| The strategies|| |
Discovery of disease-causing genes has proceeded at a faster pace after the completion of human genome project; this has broadened the scope of the further progress in gene therapy. For diseases caused by loss of function of a gene, introduction of extra copies of the normal gene may increase the amount of normal gene product to a level where the normal phenotype is restored. Thus gene augmentation therapy has been particularly applied to autosomal recessive disorder where even modest expression levels of the introduced gene may make a substantial difference. In many gene therapies the target cells are healthy immune system cells, with the idea being to enhance immune responses to cancer cells or infectious agents. Thus through gene therapy immune system-mediated cell killing is effected. In yet another strategy, delivery of gene products from cells at a remote location may be achieved. For example, myonuclei in muscle fibres have the advantage of being very long lived. Genetically engineered myoblasts therefore have the potential to ameliorate some non-muscle disease through longterm expression of transgene which encodes a protein secreted into the blood stream.
| The technology|| |
Classical gene therapy normally requires efficient transfer of cloned genes into diseased cells so that the introduced genes are expressed at a high level. For this purpose, the complete cDNA sequence of the target gene is inserted downstream to appropriate regulatory sequence in a suitable vector to ensure a high level of expression. Two general approaches are used: (a) transfer of gene into patient cells outside of the body (ex vivo) first, or (b) transfer of genes inside the body (in vitro). Ex-vivo gene transfer involves transfer of cloned genes into cells grown in culture, selection and expansion of transformed cells, and finally introduction of these transferred cells into the patient. To avoid immune rejection of the introduced cells, autologous cells are normally used. Clearly this approach is applicable only to tissues that can be removed from the body, altered genetically and returned to the patient where they will engraft and survive for a long period of time (e.g. cells of the hematopoietic system and skin cells). In-vivo gene transfer involves transfer of gene directly into the tissue of the patient. This may be the only possible option in tissues where individual cells cannot be cultured in vitro in sufficient number (e.g. brain cells, photoreceptor cells, etc.).
Following gene transfer, inserted genes may either integrate into the chromosome of the cells or remain as extra-chromosomal genetic elements (episome). The advantage of integration of the transferred gene is its perpetuation through chromosomal replication following cell division. Thus stable, long-term expression can be obtained. However, chromosomal integration normally occurs randomly which surely is a disadvantage. In some cases, however, episome-mediated gene therapy is preferred. For example, cancer gene therapies often involve transfer and expression of genes into cancer cells with the intent of killing the cells. Once the malignancy has been eliminated, the therapeutic gene may no longer be needed.
| The delivery|| |
Most gene therapy protocols have used mammalian viral vectors, as opposed to available physical techniques (electroporation, microinjection, liposome, etc.), because of their high efficiency of gene transfer. The method chosen for gene transfer depends on the nature of the target tissue and whether transfer is to be made to cultured cells ex vivo or the cells of the patient in vivo. Viruses used as vectors in gene therapy are genetically disabled (replication incompetent), and the most commonly used ones include retro-, adeno-associated, herpes, and lentiviruses. Retrovirus, an RNA virus, is one of the potent vectors for therapeutic use since it is capable of integrating into the host genome at a particular site, thus providing the potential for stable expression. However, this virus can only infect dividing cells and so cannot be used to infect terminally differentiated cells such as neurons, and is limited in its use to in vivo gene transfer. Due to their ability to transduce a wide variety of cell types in a cell-cycle independent fashion, adenovirus (a DNA virus)-based vectors have received considerable attention in recent years as delivery vehicles for multiple gene therapy applications. However, effective use of these vectors has been hampered by induction of strong immune response, acute and chronic toxicity caused by the vector, and transgene expression for a short period of time (up to 4 months) (J. Ray, unpublished data). Despite these limitations, these vectors have been used in a number of human clinical trails, eliciting contradictory results. Adeno-associated virus (AAV), an ssDNA virus, has generated considerable interest recently since it is not associated with any pathology in humans (40% of the population is seropositive for AAV without any ill effect). In addition, recombinant AAV vectors are deleted for all virally encoded proteins, therefore reducing their immunogenicity. Long-term gene expression has been achieved in several animal models using this virus. Herpes simplex virus, a large dsDNA virus, can accommodate large genes (up to 30 kb). It can infect only neuronal cells and express for a very short period. It can be applied in neuronal diseases like Parkinson's disease and to treat CNS tumours. Lentiviral vector, which is derived from HIV virus, can infect cells both at mitotic and post-mitotic stages of the cell cycle, integrates in the host chromosome and provides stable expression. Thus, this vector has lately gained considerable attention in gene therapy studies.
| Animal studies|| |
Gene therapy has been attempted in several genetic diseases including ocular, neurological, cardiovascular, metabolic, immunological, and bleeding disorders. Studies on animal models of human diseases show great promise. The most significant among these in recent years is successful restoration of vision in the dog model of Leber congenital amaurosis (LCA), a severe human retinal degeneration causing near total blindness in infancy.
| Human clinical trials|| |
The first inherited disease approved for treatment with gene therapy was adenosine deaminase (ADA) deficiency. ADA-deficient children suffer from severe combined immunodeficiency and are prone to repeated serious infections. Its first approval for gene therapy has been prompted by the fact that (a) the target gene is small, (b) targeted T-cells which are easy to culture are easily accessible enabling ex vivo gene therapy, and (c) expression of the gene is not tightly controlled. Since the pioneering work on ADA deficiency, gene therapy trials have been initiated for a few inherited disorders.
Familial hypercholesterolaemia (FH) results from the dominantly inherited deficiency of low density lipoprotein (LDL) receptors, which are normally synthesised in the liver, and is characterised by premature coronary artery disease. The first and the only gene therapy for FH was initiated in one of five homozygous affected patients in 1992. Cultured hepatocyte cells were infected in vitro by retrovirus containing LDLR gene, infused back to liver of the patient. Duchenne muscular dystrophy is a severe X-linked recessive disorder. Affected males suffer progressive muscle deterioration, are confined to a wheelchair in their teens and die usually by the third decade. Because of the unique cell biology of muscle, cell therapy has been attempted. Cystic fibrosis is an autosomal recessive disorder that results from a mutation in CFTR, which encodes a cAMP regulated chloride channel. Patients suffer from chronic infections because of the accumulation of sticky mucous in the lung. Adenovirus or liposome-mediated suitably sized CFTR minigene was tried. Despite considerable research CFTR gene therapy has so far remained largely ineffective. Cancer treatment has been attempted by gene therapy. Attempts include: (i) substitution of a functional copy of a gene for an inactive or defective gene. This technique could be used to restore the ability of a defective gene (such as p53) to suppress or block the development of cancer cells. (ii) injection of cancer cells with a gene that makes them more sensitive to treatment with an anticancer drug. Scientists hope that treatment with the drug will kill only the cells that contain the drug-sensitive gene.
| The risks|| |
Viruses usually infect more than one type of cell. Thus, when viral vectors are used to carry genes into the body, they might alter cells other than those intended. The new gene might be inserted in the wrong location in the DNA causing cancer or other damage. The transferred genes could be over-expressed to the extent that they harm the system. Also, the viral vector could cause inflammation or immune reaction, and the virus could be transmitted from the patient to other individuals or into the environment. However, scientists use animal testing and other precautions to identify and avoid these risks before any clinical trials are conducted in humans.
| The challenges|| |
The major challenge relates to finding easier and better ways to deliver genes to the body and to develop vectors that can be injected directly into the patient. These vectors must then focus on appropriate target cells (such as cancer cells) throughout the body, successfully, and integrate the desired gene into the DNA of these cells. In addition, the expression of the transplanted genes must be precisely controlled by the body's normal physiologic signals.
| The ethics|| |
At present only somatic gene therapy is done. Patients who are selected for such treatments have debilitating, often life-threatening diseases for which no effective conventional therapies are available. Appropriate regulations are in place for conducting such studies. But in future, when innovations make gene therapy protocols simpler and more accessible, the prospect of human germ-line gene therapy would be brighter, bringing with it a number of ethical concerns. Germ-line gene therapy would forever change the genetic make-up of an individual's descendants. Thus, the human gene pool would be permanently affected. Although these changes would presumably be for the better, an error in technology or judgement could have far-reaching consequences. However, germ-line therapy is not yet an approved area of research.
| References|| |
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